Comment on “ferroelectricity-free lead halide perovskites” by A. Gómez, Q. Wang, A. R. Goñi, M. Campoy-Quiles and A. Abate, Energy Environ. Sci., 2019, 12, 2537

This article comments on the recent publication “Ferroelectricity-free lead halide perovskites” by Gomez et al. [DOI: 10.1039/c9ee00884e], in which the authors conclude that both methylammonium lead iodide (MAPbI3) and the more advanced Cs0.05(FA0.83MA0.17)0.95Pb(I0.83Br0.17)3 form non-ferroelectric thin-films. This conclusion is based on measuring the vertical piezoelectric effect by “direct piezoelectric force microscopy” (DPFM) and not seeing any domain patterns or other ferroelectric responses. The authors calibrated their measurement using a bulk reference sample of periodically poled lithium niobate with vertical polarization, which has all-different properties from MAPbI3 or Cs0.05(FA0.83MA0.17)0.95Pb(I0.83Br0.17)3 thin-films. In earlier works, it was pointed out that the polarization in large MAPbI3 grains is vastly oriented in-plane and hence could remain invisible to any probing techniques with vertical sensitivity. In addition, the low spatial resolution of their measurements, the strong measurement noise, potential adventitious water contamination and the use of improper cantilever loads reduces the sensitivity of the measurement setup. This is why the conclusion on MAPbI3 being non-ferroelectric is not supported by the measurement data.